This issue has been mentioned so many times (or variants of it; e.g. port resets itself over time), but none if this works with new Ubuntu Server 20.04 VM for me.
Trying to open TCP ports 80 and 443, but the only one that gets opened is TCP 22 that was specified during creation. None of the following workarounds work:
delete and re-create rule with same values
dissociate/associate NSG
create new NSG with exact same value, dissociate original NSG, and associate the new one
Waited at least 30 mins after each workaround, no joy. I also don't think that rule propagation is the issue, because if I enable ICMP, then pinging works immediately.